A problem frequently attending the application of more or less viscous compositions to a carrier material is a change in certain properties, especially rheological properties, of the composition in the course of processing and in particular in the course of application. Such changes may occur as a result, for example, of a temperature change, of an increase in the solids content owing to evaporating solvent, or of the progress of a chemical reaction during application of the composition. These changes in property are almost always a critical parameter for the quality of the coating, especially for the uniformity of the coatweight. Consequently, if the aim is to achieve an even coatweight over the entire coating width, either the state of the composition must be the same across the entire width, or it must be possible for non-uniformities to be specifically compensated during the coating operation.
Application apparatus known in the prior art and producing a rotating body of composition occasionally also called the bank have to date lacked sufficient suitability to counter the problem of viscosity parameters changing during application. With these technologies, instead, the tacit assumption is made that the rheological properties of the material to be coated (composition) are unchanging during the coating operation. In actual fact, however, the composition is generally supplied from a tube or an extruder to the coating assembly at a location in open form, or undergoes rough preliminary distribution by means of distributor pipes, oscillating feeder strips, or slot dies, for example. Finally, the composition is shaped to a precise layer in multi-roll applicators, in calenders or at coating bars or doctor blades, and is transferred to a carrier material, the composition becoming built up at at least one of the distributing doctor blades or rolls. Between rolls at different speeds or between doctor blade and web to be coated, or between doctor blade and rotating roll, a rotating bead of composition is developed, referred to as the body of composition, or alternatively as the bank. Both in the preliminary distribution and in the rotating body of composition, the individual volume elements of the composition are held up for different durations by different pathways, flow profiles and backflows. Changes which occur to different extents locally here in composition properties such as temperature, reaction status or solvent content cannot be compensated, or can be only partly compensated, at the coating assemblies.
Slot dies, for example, having comparatively broad residence time distribution, meaning that the flow of composition at the die edges is much older than in the middle, and rheological changes contingent on chemical reactions have therefore often proceeded further at the edges than in the middle. If, then, the flow of composition is to be coated from a slot die via a calender applicator, the overlapping of different effects in the calender results in further irregularities. Only at the locations at which the skein of composition emerging from the die is thicker than is needed does the excess feed the rotating body of composition. Conversely, at the locations at which the skein of composition is too thin, the absent composition is made up from the rotating body of composition. In the body of composition, then, there is a redistribution of fractions of composition, and this is associated with additional residence time. Optimized preliminary distribution of the composition with the die in fact extends the residence time of the composition in the rotating body of composition, and this residence time may well exceed the pot life. This is the reason why ever-improved regulation of the die lip (automatic die) is unsuccessful, and instead, on the contrary, makes the situation worse. A mathematically perfect preliminary distribution of the composition, which would circumvent this problem, is virtually impossible.
As a result of the overlapping of the two effects, it is impossible overall to calculate the age of the composition transverse to the machine direction in application processes of this kind. The locations at which older composition is coated form thick points within the finished product, known as piston rings, thereby detracting from product quality.
There are a number of examples known in the patent literature for improving the processing of polymer material streams of relatively high viscosity. For instance, GB 1,158,890 B5 describes a 2-roll mill for producing films up to 0.5 mm thick, with supply of composition on both sides, as a cheaper alternative to roll mills having three to four rolls. Here, initial shaping of the layer takes place between the end zones of the rolls; subsequently there is a transfer of the flow of composition into an isolated middle zone of the rolls, by means of “stopping elements”, and the film is shaped.
GB 1,190,245 B1 describes a 2-roll mill with unilateral supply of composition for the removal of ethylene glycol from a high-viscosity composition during the production of PET by polycondensation. Here, the continually renewed surface of composition on the hot rotating rolls is utilized for more rapid evaporation of the ethylene glycol.
It is an object of the invention to provide a method for applying a flow of composition to a carrier material that allows even coating to be achieved with formation of a rotating body of composition. Of particular intention is that the coatweight of the applied composition should as far as possible be the same over the entire area to be coated.
The achievement of the object is achieved through the fundamental concept of the invention, whereby the composition is supplied not in the usual way in open form at a location or in roughly predistributed form more or less uniformly on the full width of the web to be coated, by means of distributor tubes, oscillator feeder strips or slot dies, but instead from the side.